As is well known, an electric tool such as a drill-driver sometimes causes that the output shaft thereof is locked up while using the tool. At this time, the motor as a drive source thereof is also locked up (motor-lock). The motor does not rotate during the motor-lock. Therefore, temperature of the winding wire of the motor increases if the motor-lock continues, and the motor may be caused a thermal damage. In this case, the performance of the motor may be seriously deteriorated.
Therefore, it has been demanded to detect as soon as possible the occurrence of the motor-lock in order to decelerate the motor or stop the supply of electric current.
The occurrence of the motor-lock can be detected, for example, by monitoring the rotary speed of the motor, measuring the motor current, or measuring the temperature of the motor. However, these configurations require additional sensors. Incidentally, it has been known that the voltage of the battery decreases during the motor-lock because large electric current flows, and therefore the motor-lock can be detected by utilizing this characteristics. Herein, the electric tool that is powered by a secondary battery of a lithium-ion battery is generally configured to stop the supply of the electric current to the motor when the battery voltage drops to a certain voltage (hereinafter, referred to as “overdischarge protection voltage”), in order to prevent an overdischarge of the lithium-ion battery. The occurrence of the motor-lock can be prevented by this configuration. That is, when the battery voltage drops to the overdischarge protection voltage due to the voltage drop caused by the motor-lock, the electric tool stops supplying the electric current to the motor. Therefore, this “overdischarge protection function” may serve for preventing the continuation of the motor-lock.
As for an electric tool that is provided with a comparatively large motor, a large electric current of 100 [A] or more flows during the motor-lock. In this case, the battery voltage drops to reach the overdischarge protection voltage during the motor-lock. On the contrary, as for an electric tool that is provided with a comparatively small motor, the electric current that flows during the motor-lock is several tens of amperes, and therefore the battery voltage may not reach the overdischarge protection voltage even when the motor-lock occurs. In this case, the motor cannot be protected.
Incidentally, Japan patent document 3268086 B discloses a protection circuit that includes a comparator and that is configured to detect the occurrence of the motor-lock based on an amount of voltage drop within a predetermined period of time. This circuit can detect the occurrence of the motor-lock if the battery voltage changes rapidly. However, this circuit cannot detect the occurrence of the motor-lock if the battery voltage changes slowly as in a case of tightening a wood screw having a large diameter and a long length, or a case of boring a hole of a large diameter.